Sounding reference signal (SRS) is an uplink reference signal transmitted from a terminal device (e.g., a user equipment (UE)) to a network device (e.g., a base station). A SRS may be used by the network device to determine the channel quality of the uplink path from the terminal device to the network device. The channel quality determination may be referred to as channel estimation. The network device often allocates a portion of system bandwidth for a specific terminal device at a specific time, and the network device may select the best bandwidth for the terminal device based on the SRS transmitted by the terminal device.
The network device may also use SRS for uplink timing estimation as a part of timing alignment procedure, particularly when there is no physical uplink shared channel (PUSCH) or physical uplink control channel (PUCCH) transmissions for an extended period.
Several types of SRS transmissions are defined in literature. For example, the long-term evolution (LTE) defines single SRS transmissions, periodic SRS transmissions, and aperiodic SRS transmissions. The network device typically provides type(s) and timing of SRS transmissions to the terminal device, and the information provided by the network device regarding SRS transmission is often referred to as SRS transmission configuration.
SRS transmission configuration may dictate that SRS be transmitted at a specific position in a signal transmission hierarchy, e.g., the last symbol of an uplink slot. In some scenarios, a terminal device may transmit a SRS only when the terminal device wins a resource competition in a contention-based protocol (e.g., listen-before-talk, LBT). In these and other scenarios, such singular SRS transmission configuration may be inefficient.